1. Field of the Invention
The present invention relates to the recovery of paper mill waste sludge. More particularly, the present invention relates to the production of high heating value fuel products from the dredged and dewatered paper mill sludge.
2. Description of Related Art
Paper is conventionally made by draining a low consistency dispersion of cellulose fiber pulp, fillers and additives through a paper machine xe2x80x9cwirexe2x80x9d (essentially an endless mesh or sieve). A certain amount of solid material passes through the wire with the suspending water, thus it is not retained in the wet paper web formed on the wire. The drained suspension water and suspended solid material is known as xe2x80x9cwhite waterxe2x80x9d and is reused as far as practicable. Complete recovery and reuse of water and papermaking raw materials is impossible, and a certain proportion is discharged as effluent. Waste also occurs when the type or grade of paper being produced alters, especially if this involves a color change or the use of different additives or fillers.
Effluent from the paper machine has to be treated before it can be discharged from the mill. This treatment normally involves passage through a clarifier, prior to which flocculants are added to promote sedimentation of solid material. A biological treatment with microorganisms is often carried out in order to reduce the biological oxygen demand of the liquid effluent before it is discharged.
The sediment which accumulates is disposed of as a sludge composed of pulp fibers, fiber particles (xe2x80x9cfinesxe2x80x9d), fillers and pigments, together with miscellaneous debris such as grit, sand, plastic particles, general dirt and, particularly if waste paper has been used by the mill as a raw material, ink particles, accumulations of adhesives and foreign bodies. The paper mill sludge is accumulated in the form of a pond or kept within a large open-air tank. Paper mill sludge has substantially little use as a material that can be employed in any other industrial applications. Because of its non-utility, the paper mill sludge is merely discarded, along with other waste cellulosic fiber, creating a tremendous disposal problem.
In certain circumstances, the sludge is drawn off from the pond or tank at about 2.5 percent consistency and then is dewatered to a consistency of around 20 to 25 percent, by means of rotary vacuum filters. It is then semi-solid and is collected for transport and disposal, such as into landfill sites. This is expensive, since a medium to large paper mill can generate thousands of tons of sludge for disposal each year. Landfill and associated transport costs can be expected to increase as landfill sites become scarce.
Attempts have been made to find economic uses for paper mill sludges which avoid the need for landfill or other disposal, and/or to recover reusable raw materials from the sludges. Some sludges can be burned for steam and/or power generation, but the practicality of this depends on the nature, amount and variability of the sludge produced. For example, sludges with a high filler content, as might be produced in a paper mill specializing in fine paper production, may not be adequately combustible, and/or the amount of sludge available may not justify investment in a suitable combustion plant or adaptation of existing plants.
The paper mill sludge can contain toxic chemicals and elements within the waste cellulosic fiber. The products typically containing dioxins are a major source of toxic contamination of soils and water supplies. Dioxins are suspected of causing cancer and birth defects. From the beginning of the industrial age, large quantities of materials containing dioxins have been dumped, either accidentally or deliberately, in a heretofore entirely reckless fashion. Consequently, material containing dioxins is a significant hazard to the environment. The disposal of all materials containing dioxins, and particularly paper mill sludge, has caused great concern. Incineration has been employed as a means of disposal and land burial has been the other primarily practiced method of disposal of waste cellulosic fiber and paper mill sludge.
It is an object of the present invention to provide a process which effectively converts the paper mill sludge into a usable fuel product.
It is another object of the present invention to provide a process whereby the paper mill sludge can be suitably pelletized for transportation and burning.
It is another object of the present invention to provide a process for forming a fuel product which produces a fuel product having a heating value of greater than 5,000 BTU/pound from paper mill sludge.
It is another object of the present invention to provide a process for treating paper mill sludge which effectively removes the harmful effects of dioxins.
It is still another object of the present invention to provide a process for making a high heating value fuel product from paper mill sludge which is easy to use, relatively inexpensive and economically beneficial.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.
The present invention is a process for making a fuel product from paper mill sludge comprising the steps of: (1) dewatering the paper mill sludge so as to have a solids content of greater than 7 percent by weight; (2) mixing an oxide-containing chemical and molasses with the dewatered paper mill sludge; (3) pressurizing the mixed sludge to a pressure of greater than 6 p.s.i. for a period of time of no less than 15 seconds; and (4) drying the pressurized mixed sludge to no less than 60 percent solids by weight.
Within the process of the present invention, the paper mill sludge is dredged prior to the step of dewatering. The oxide-containing chemical can be either calcium oxide or calcium hydroxide in an amount of between 1 to 10 percent by weight of the dewatered paper mill sludge. The molasses is mixed in an amount of between 0.1 percent and 1 percent by weight of the dewatered paper mill sludge. The step of pressurizing is carried out between 6 p.s.i. and 500 p.s.i.
Within the process of the present invention, the pressurized mixed sludge is passed to a flashing chamber and then to a drying field. The dried sludge can then be ground to a desired mesh size of no less than 325 mesh and no larger than one-quarter inch. The dried sludge will have a heating value of no less than 5,000 BTU/lb.
The step of pressurizing includes passing the mixed sludge as a flow through a pipe. The pipe maintains the mixed sludge at a pressure of greater than 6 p.s.i. The pipe has a length such that the flow of the mixed sludge takes longer than 15 seconds to pass through the pipe. The pressurized mixed sludge can then be flashed from the pipe through an orifice in the pipe into a flash chamber having a pressure therein of less than 6 p.s.i.